Natural gas liquefaction cycles

Constantinos Hadjistassou, PhDAssistant Professor

Programme in Oil & Gas (Energy) EngineeringUniversity of Nicosia

Web: www.carbonlab.eu

Nov., 2015

Overview

Liquefaction cycles: 1) Joule-Thomson cycles 2) Expander cycles 3) Cascade cycles for NG liquefaction

Vapor-compression refrigeration cycle

Classical cascade cycle

Mixed refrigerant cascade (MRC) cycles

Thermodynamic calculations of LNG processes

Choosing among liquefaction cycles

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LNG plant flow diagram

Delivered gas stream at about 1,300 psi NG is metered before pre-treatment Cryogenic cooling (<−150˚C) usually accomplished in several steps Flash vapors either recycled or used as a fuel Most LNG contracts specify a range of acceptable heating values

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What is an LNG train?

“Trains” treat & liquefy natural gas and store it in LNG tanks Train size governed by: liquefaction process, refrigerant(s), size of

compressor/driver combo, size of heat exchangers

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Recap: Joule-Thomson & Expander cycles

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Joule-Thomson cycle

Closed-cycle Open cycle liquefaction

Recall advantage of (nearly) reversible (ideal) gas expansion over J-T: – A large part of compression work can be recovered thru heat exchanger higher η– Reversible process attains larger cooling effect

Cascade Cycles

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Some definitions

Thermodynamic cycles consist of power cycles & refrigeration cycles

Refrigeration is the transfer of heat from a lower temperature to a higher temperature reservoir, facilitated by mechanical work.

What is a refrigerant? It is the working fluid used in a refrigeration cycle.

Sensible heat is the amount of heat required to raise the temp. of a unit mass of a substance by 1 K, (Q = mcpΔΤ; cp, H2O = 4,181 J/kgK @ 20°C)

Latent heat is the thermal energy released or absorbed by a body during a phase change at constant temperature eg melting of ice or boiling H2O. Lvaporis. = 2,260 kJkg-1 | Lfusion = 334 kJkg-1

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Some definitions

Entropy (s) is the notion tied to the 2nd Law of Thermodynamics. It is defined as the “degree of disorder of a thermodynamic system” or “specific ways in which a thermo system can be rearranged.” Stm at equilibrium has max. S. S units: J/K

Enthalpy (h) is the thermodynamic potential of a system:

where u is the internal energy, p is pressure & v is volume. h units: J

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RevdQsT

h u pv

Carnot refrigeration cycle

Profound societal & economic impacts: Changed the way we process food Expanded agricultural markets & human diet Air-conditioning improves thermal comfort, productivity, standard of living

Closed loop cycle whose working medium is vapour & liquid Processes are internally reversible Latent heat absorbs & rejects heat btw colder & hotter environments

1→2: isentropic compression 2→3: isothermal condensation 3→4: isentropic expansion

through turbine 4→1: isothermal evaporation

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Carnot vapour refrigeration cycle

Refrigeration

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Vapour compression refrigeration

Change in phase (or mix %) facilitates heat absorption & rejection In an actual vapor-compression refrigeration cycle turbine replaced by

expansion valve In reality, irreversibilities lower performance. Mismatch btw cold & hot regions and refrigerant

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Actual vapor-compression cycleVapor-compression refrigeration system

Distinction btw refrigerators & heat pumps

A refrigerator and a heat pump transfer heat from a low-temp. reservoir to a higher temp. region

They are the same devices but differ in their use A refrigerator cools a cold space A heat pump warms a space (house) Both require mechanical work Heat pump works on principle of operation of

modern air-conditioning systems

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How do refrigerators work

1. Vapour. Starting the cooling cycle, the refrigerant is in gaseous state

2. Compressor. A compressor then pressurises the vapour, warming it up

3. Condenser. Once hot & highly pressurised, the vapour is channelled outside the fridge into a condenser, where it is cooled into a high pressure liquid

4. Expansion valve. In liquid form the refrigerant is drawn in an expansion valve & back into the low-pressure fridge compartment

5. Compartment. As the refrigerant vaporises it cools the main compartment of the fridge before being directed into the compressor to begin the cycle all over.

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Cascade cycles

Maximum thermodynamic efficiency of liquefaction cycle attained if: “the heating curve of refrigerant matches the cooling curve of NG”

Working media: 2 fluids (NG & mixed refrigerant) Thermodynamically, the mixed refrigerant emulates a reversible

process because it minimizes ΔT (Δh) Smaller ΔΤ less power per kg LNG In practice how do mimic the cooling curve? Increase heat transfer efficiency by

minimizing ΔΤ btw refrigerant & NG Use a series of refrigerants (usually 3) is

a process known as a cascade cycle >3 refrigerants can be used but with

added costs & complexity

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−161°C = −258°F

Cascade cycles

1. Classical cascade cycle 2. Mixed-refrigerant cascade cycle

Closed cycle Open cycle

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Natural gas liquefaction cycles

Liquefaction cycles: 1. Classical cascade

Refrigerants: a) propane, b) ethylene, c) methane in compr.-refrig. cycles

2. Modified cascades: Mixed refrigerant

Fewer compressors & heat exchangers Less space Less costly to build Costs less to operate

Precooled mixed refrigerant Most popular cycle Uses mixed refrigerants: N2, CH4, C2H6, …. Known as C3 MR cycle

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1. Classical cascade

Fig. 1 illustrates a two working fluid cascade cycle Working fluid compressed (1) and then cooled in heat exchanger E-1 Liquid from R-1 enters E-2 where it is cooled & expands thru J-T valve Vapor directed thru E-1 Liquid from R-2 cools E-4 gas Temp:

R-1>R-2>R-3>R-4>R-5>R-6 Feed gas progressively

cools until it is expanded through a J-T valve & separated from its vapor

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Fig 1. Two-fluid cascade cycle

Classical cascade (2)

NG liquefaction uses 3 refrigerants with 2 or 3 refrigeration stages Refrigerants are pure fluids (m-e-p):

1) propane, 2) ethylene (or ethane) 3) methane

Heat exchangers are cooled by refrigeration cycles

For fluids to match part of their vapor pressures must overlap

Vapor (or equilibrium) pressure Note: use of 3 compressors is not

favorable

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High vapor pressure =

volatile

2. Mixed refrigerant cascade

Principle of mixed-refrigerant cascade (MRC), auto-refrigerated cascade (ARC) & one flow cascade (OFC)

Aims to mimic the NG cooling curve by using 1 refrigeration loop Refrigerant: single fluid whose components liquefy @ varying temps Attains closer match btw NG cooling & refrigerant heating Only one compressor is needed

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Mixed refrigerant cascade

Open cycles Closed cycles

2. Mixed refrigerant cascade: closed cycle

Refrigeration mixture = f(NG composition)

Refrigerant mix may consist of N2, CH4, C2H6, C3H8, C4H10, & C5H10

Working fluid subject to pressure drops & liquid-vapor separations

Temp of NG gradually reduced to −161°C

Refrigerant & NG do not mix

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H2O cooled condenser

Heat exchangers

Propane pre-cooled mixed refrigerant (C3MR)

Most common mixed refrigerant stm for baseload NG plants

Employs external propane refrigeration & mixed fluid J-T expansion

Fluid #1: propane, #2: mixed refrigerant, #3: treated NG

Uses two compressors

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Propane cooling system

2. Mixed refrigerant cascade: open cycle

NG stream mixed with refrigeration mix

Mixing can be done: before, during or after compression

Final separator divides NG into LNG & flash gas

Flash gas used to power LNG plant

Use of a single compressor is advantageous

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Heat exchangers

Mixed refrigerant cascades

Pros of MRC: – Use 1 compressor means:

less complexity simple piping network Simplification in instrumentation

– Ability to alter the composition of the refrigerant mix – Adjusting the refrigerant mix promotes process optimization – Ease of extracting refrigerants from NG stream

Cons: – Facilities for recovering, storing, & blending of refrigerants – Refrigerants are flammable but so is NG

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Coolers (heat exchangers)

Two main types of heat exchangers (HE): coil wound heat exchangers & plate-fin HE

Coil wound (CWHE) can measure 5m in diameter & 70 m in length while weighing 300 tonnes

Consist of thousands meters of tubing & can handle press. of 1,100 psig Note that it can take up to 30 months (ordering to installation)

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Plate-fin heat exchangers (PFHE)

Plate-fin HEs are less expensive than CWHE PHFE fabricated by more manufacturers (than CWHE) More popular than CHWE (because of few manufacturers for CWHE)

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Thermodynamic calculations

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Ideal cooling of NG (methane)

Using the energy balance and mass conservation, the cooling load (Q) is

where Qcool is the cooling load (J or BTU), min & mout is the mass in & out, hin & hout is the enthalpy in & out (J/kg) Since min=mout then min=mout=m. Therefore, eqn(1):

: heat (cooling load)/unit mass (kJ/kg) [Unit cooling load] ‘Newton’s law of cooling’ states that for a constant heat transfer

coefficient (U), the rate of heat loss of a body is proportional to the difference in temperature btw the body & its surroundings.

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(1)cool out out in inQ m h m h

ˆ (2)coolcool

QQm

ˆcoolQ

( ( ) ) ( ) (3)envdQ hA T t T hA T tdt

Ideal cooling of NG (CH4)

Heat transfer coefficient (hc) is the proportionality coefficient btw heat flux (heat flow per unit area, q=Q/A) & the thermodynamic driving force for the flow of heat (ie, temp. gradient, ΔT)

Heat transfer during gas cooling is:

where Uc is overall heat transfer coefficient (W/m2·C) and is the heat flux per unit mass (kJ/kg)

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ˆ (4)cool cQQ U A Tm

ˆcoolQ

Ideal vapor-compression cycle (2)

Distinction btw reversible & ideal V-C cycles: the ideal VC cycle is not internally reversible

Refrigerant: ethyl ether Refrigerator:

Heat pump:

where h1=hg@P1 & h3=hf@P3

h values read-off from refrigerant tables (eg, R-134a)

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L 1 4R

Net,in 2 1

q h hCOPW h h

2 3HHP

Net,in 2 1

h hqCOPW h h

Actual vapor-compression cycle

In reality, irreversibilities plaque the ideal VC cycle Fluid friction (induces Δp) Heat transfer to & from the surroundings

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Slightly superheatedvapor

Coefficient of performance (COP)

The efficiency of a refrigerator is expressed in terms of the coefficient of performance (COP) denoted by COPR:

(6) The COP of a refrigerator (or heat pump) which operates on the

reversed Carnot cycle is termed a Carnot refrigerator (or a Carnot heat pump). The refrigerator efficiency is given by:

where QL is amount of heat absorbed from the low-temperature medium & QH is the amount of heat rejected to the high-temp medium. For a reversible refrigerator:

(7)

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Example #1

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Choosing among liquefaction processes

Note that several of “off the shelve” refrigeration cycles are licensed Licensors include: Shell, ConocoPhillips, Air Products CI Complicated exercise which factors in:

LNG plant throughput (e.g., MTPA) Local (ambient) conditions Gas composition & reserve size

Trend towards large “trains” (8 mtpa≈ 1.2bcf/d)

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Choosing among liquefaction cycles (2)

Nearly always trade-off between CAPEX & plant efficiency Ease of start-up Ability to manage changes in feedstock compositions Maintenance costs Safety considerations Previous experience (where it exists)

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NG liquefaction costs

≈10% of the NG received by an LNG plant is used to condense the gas Refrigeration & liquefaction account for:

About 35-45% of CAPEX (20-30% of total project costs incl. regas + LNG ships) Around 50% of the plant operating costs

Why is it so expensive? High cost of steel Often LNG plants are built at remote locations Strict design & safety levels Large volumes of refrigerants are used Redundancy alleviates risk of breakdowns but comes at a cost Greenfields (vs. brownfields) come at a higher cost

Cost offsets: Exploit economies of scale i.e., larger liquefaction

plants

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Thanks for your attention!

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